1. Field of the Invention
The present invention relates to a method for making a pulp and catalyst, and particularly to a method for making a metal/titania pulp and photocatalyst.
2. Related Art
Use for photochemical reaction and application in industry, of semi-conductor materials, are hot research directions in recent years, mainly because special optical properties of the semi-conductor materials make wastewater treatment or energy regeneration such as hydrogen production with solar energy to become feasible. However, as a catalyst effective in photochemical reactions, a photocatalyst can absorb photo energy and thus produce electron-hole pairs only on condition that a band gap between valence band and conduction band of the photocatalyst matches with the spectrum of incident light, and the photocatalyst can be used in a catalysis, oxidation, and reduction reaction to decompose relevant compounds only on condition that the band gap matches with electric potential relevant to a chemical reaction.
Photocatalysts such as TiO2, ZnS, Fe2O3, and ZrO2 have a common property, that is, they have a wide enough energy band gap and a high enough water decomposition activity. These semi-conductor photocatalysts will have photochemical activity and capability generally only after irradiation with UV light. However, UV light is present in solar light spectrum in a low proportion of 4%, therefore, how to use solar energy effectively seems to be an important subject at present. Because visible light is present in an amount of about 43% in solar light spectrum, now scholars in each field are actively developing a photocatalyst with high activity in visible spectrum for use in catalysis of decomposition and redox reactions to explore a new way of solar energy utilization. Among relevant semi-conductor photocatalyst materials, most frequently used one is titania, mainly due to its stable properties, non-toxicity, low cost, ease availability, and good photochemical activity. However, its absorption spectrum for solar light still concentrates in UV region, so in order to more efficiently absorb solar light of visible wavelengths by titania, titania is suitably modified to achieve a wider absorption range for solar light spectrum, which will become a feasible method to efficiently improve the utilization rate of solar energy.
Titania has been widely used in various industrial applications including dyestuff, paper industry, paint, catalyst, bactericide, detergent, surface treatment, wastewater treatment, and decomposition of organic waste, etc. Recently, titania is also gradually used in advanced technology industry due to its special semi-conductor properties. Titania belongs to n-type semi-conductors and has a molecular structure of zinc-blende lattice, which can be divided into three main crystal forms, i.e. anatase, rutile, and brookite.
Generally, titania is of amorphous structure at normal temperature, and it will be present as anatase crystal form upon calcination at a temperature between 200° C. to 500° C., as rutile crystal form at a temperature between 500° C. to 600° C., and as brookite crystal form where the calcination temperature is above 700° C. Anatase and rutile will vary with temperature, so they are often used in photocatalytic reaction. The rutile crystal form is more stable, while the photo-reaction activity of anatase is better; therefore, anatase is commonly used as main raw material in many industrial applications. Because titania has an excellent photocatalyst activity and a band gap between valence band (VB) and conduction band (CB) is up to 3.0-3.2 eV, the incidence of light with an energy higher than this band gap on titania will lead to separation of electron-hole pairs, and the generated electrons and holes will recombine too. The separation and recombination of electron-hole pairs are competitive mechanisms, and where there only occurs separation of electron-hole pairs and electrons are concerned in free radical reactions respectively, the photocatalytic activity can be exhibited.
It can be found from studies in literatures that titania powders produced with different preparation methods will have different surface properties, including particle size, porosity, particle structure, and morphology etc, all of which will influence optical activity of titania. Generally, the optical activity of titania will directly affect its efficiency, for example, for decomposition and breakdown of organic components in wastewater treatment, and for electron transfer on thin film electrode in a dye sensitized solar cell.
Recently, because nano-titania powder has been widely used in various industries, and demand is continuously increased, many commercial processes for producing titania powder in large scale are successively developed, for example, Degussa P25. However, the nano-titania powder is very fine, for example, when used in an aqueous solution system for degrading organics contained therein, it is difficult to separate the nano-titania particles suspended in the solution from the aqueous phase after the reaction is complete, limiting its applications. In order to solve such a problem, formulating the prepared titania powder into a pulp and applying it onto a substrate to prepare a titania film is another feasible solution.
Methods to produce nano-titania powder generally can be divided into two general classes, i.e. first class of liquid-phase synthesis and second class of gas-phase synthesis. The first class of liquid-phase synthesis can be further divided into (1) sol-gel process: a high purity of metal alkoxide (M(OR)n) or metal salt is dissolved into a solvent such as water or alcohol, hydrolyzed, and condensed to form a gel, thereby producing a gel having several spatial structures; (2) hydrolysis process: a metal salt is subjected to forced hydrolysis in different acidic or alkaline solutions to produce uniformly dispersed nano-particles; (3) hydrothermal process: a reaction is preformed in a sealed stainless steel vessel at a particular temperature and under a particular pressure to produce nano particles; and (4) microemulsion process: a titanium-containing precursor is added into a microemulsion of water with surfactant to form a nearly monodispersed nano-sized micelle, and then dried and calcinated.
The second class of gas-phase synthesis can be divided into (1) chemical vapor deposition: a precursor is reacted with oxygen in a low-pressure chemical vapor deposition apparatus to produce a thin film or a powder; (2) flame synthesis: a metal compound supplied by a system is vapor heated with oxyhydrogen flame or oxyacetylene flame etc to produce nano particles; (3) vapor condensation: a raw material is gasified or formed into a plasma by a heating process such as vacuum evaporation, heating, or high frequency induction, and then quenched to collect a nano powder produced; and (4) laser ablation: a metal or non-metal target is gasified by a laser beam of high energy and then the vapor is condensed to obtain a stable atomic cluster in gas phase.
As described above, absorption wavelengths of solar light by the titania photocatalyst alone mainly concentrate in UV region, therefore, binding to different metal ions is a feasible means in order to improve its light absorption range, photocatalysis, reduction, and oxidation activities. Previous studies in literatures mainly focus on preparation of a desired catalyst powder by dipping, in which the prepared titania powder is soaked into different metal ion solutions, or a metal ion solution is dripped into the titania powder. In the photocatalyst powder thus prepared, generally all the metal ions are physically adsorbed or attached to surface of the titania powder, therefore, the binding strength between the metal ions and the titania powder is low and disassociation may easily occur during the reaction process, resulting in decreased light absorption and reaction effects. In order to improve such a disadvantage, the present invention utilizes a unique reaction manner in which one or more metal ions are added in a titania reaction process such that they can effectively bind to titania in the formation, whereby modified properties and improved photochemical catalytic activity of simple titania photocatalyst can be achieved, realizing its effective application in industry.